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T5761-TG Datasheet(PDF) 5 Page - ATMEL Corporation |
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T5761-TG Datasheet(HTML) 5 Page - ATMEL Corporation |
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5 / 32 page  T5761 T5760 / Preliminary Information Rev. A2, 19-Oct-00 5 (32) and exhibits the best possible sensitivity and at the same time power matching at RF_IN. RSens can be connected to VS or GND via a µC. The receiver can be switched from full sensitivity to reduced sensitivity or vice versa at any time. In polling mode, the receiver will not wake up if the RF input signal does not exceed the selected sensitivity. If the receiver is already active, the data stream at Pin DATA will disappear when the input signal is lower than defined by the reduced sensitivity. Instead of the data stream, the pattern accord- ing to figure 5 is issued at Pin DATA to indicate that the receiver is still active (see also figure 32). DATA t DATA_L_max DATA_min t Figure 5. Steady L state limited DATA output pattern FSK/ASK Demodulator and Data Filter The signal coming from the RSSI amplifier is converted into the raw data signal by the ASK/FSK demodulator. The operating mode of the demodulator is set via the bit ASK/_FSK in the OPMODE register. Logic ‘L’ sets the demodulator to FSK, applying ‘H’ to ASK mode. In ASK mode an automatic threshold control circuit (ATC) is employed to set the detection reference voltage to a value where a good signal to noise ratio is achieved. This circuit also implies the effective suppression of any kind of in-band noise signals or competing transmitters. If the S/N (ratio to suppress in-band noise signals) ex- ceeds about 10 dB the data signal can be detected properly, but better values are found for many modulation schemes of the competing transmitter. The FSK demodulator is intended to be used for an FSK deviation of 10 kHz ≤ Df ≤ 100 kHz. In FSK mode the data signal can be detected if the S/N (ratio to suppress inband noise signals) exceeds about 2 dB. This value is valid for all modulation schemes of a disturber signal. The output signal of the demodulator is filtered by the data filter before it is fed into the digital signal processing circuit. The data filter improves the S/N ratio as its pass- band can be adopted to the characteristics of the data signal. The data filter consists of a 1st-order highpass and a 2nd-order lowpass filter The highpass filter cut-off frequency is defined by an external capacitor connected to Pin CDEM. The cut-off frequency of the highpass filter is defined by the follow- ing formula: fcu_DF + 1 2 p 30 k W CDEM In self-polling mode, the data filter must settle very rapidly to achieve a low current consumption. Therefore, CDEM cannot be increased to very high values if self- polling is used. On the other hand CDEM must be large enough to meet the data filter requirements according to the data signal. Recommended values for CDEM are given in the electrical characteristics. The cut-off frequency of the lowpass filter is defined by the selected baud-rate range (BR_Range). The BR_Range is defined in the OPMODE register (refer to chapter ‘Configuration of the Receiver’). The BR_Range must be set in accordance to the used baud-rate. The T5760/T5761 is designed to operate with data coding where the DC level of the data signal is 50%. This is valid for Manchester and Bi-phase coding. If other modulation schemes are used, the DC level should always remain within the range of VDC_min = 33% and VDC_max = 66%. The sensitivity may be reduced by up to 2 dB in that condition. Each BR_Range is also defined by a minimum and a maximum edge-to-edge time (tee_sig). These limits are defined in the electrical characteristics. They should not be exceeded to maintain full sensitivity of the receiver. Receiving Characteristics The RF receiver T5760/T5761 can be operated with and without a SAW front-end filter. In a typical automotive application, a SAW filter is used to achieve better selec- tivity and large signal capability. The receiving frequency response without a SAW front-end filter is illustrated in figures 6 and 7. This example relates to ASK mode. FSK mode exhibit similar behavior. The plots are printed rela- tively to the maximum sensitivity. If a SAW filter is used, an insertion loss of about 3 dB must be considered, but the over all selectivity is much better. When designing the system in terms of receiving band- width, the LO deviation must be considered as it also determines the IF center frequency. The total LO devi- ation is calculated to be the sum of the deviation of the crystal and the XTO deviation of the T5760/T5761. Low- cost crystals are specified to be within ±90 ppm over tolerance, temperature and aging. The XTO deviation of the T5760/T5761 is an additional deviation due to the XTO circuit. This deviation is specified to be ±30 ppm worst case for a crystal with CM = 7 fF. If a crystal of ±90 ppm is used, the total deviation is ±120 ppm in that case. Note that the receiving bandwidth and the IF-filter bandwidth are equivalent in ASK mode but not in FSK mode. |
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